The effect of egg size on post‐hatching development in the Razorbill: an experimental study

Abstract

A recent experimental study reported that Thick‐billed Murre Uria lomvia chicks that hatched from large eggs grew their wing feathers more quickly than did small‐egg chicks. There is little evidence of this (or any other) egg‐size effect on post‐hatching development in other birds. Thick‐billed Murres are marine birds of the family Alcidae that employ the unique “intermediate” developmental strategy: chicks go to sea after 15‐30 days at the nest site, at <30% of adult mass, accompanied by their male parent. Rapid feather growth during the brief nestling period is critical to enable chicks to make the transition from life at the nest site to life at sea quickly and safely. At the Gannet Islands, Labrador, Canada, in 1996 and 1997, I tested whether egg size has the same effect on wing‐feather growth in the Razorbill Alca torda, another of the intermediate auks. To control for underlying correlations between egg size and other parental attributes, eggs were switched randomly among pairs. As in other birds, egg size strongly predicted hatchling mass, and to less extent hatchling size (tarsus length), but had no effect on the rate at which nestlings gained mass. However, egg size had the same effect on wing growth in Razorbills as in Thick‐billed Murres: the wings of large‐egg chicks began rapid, linear growth sooner, indicating that early development of wing feathers was enhanced in large‐egg chicks. Differences in wing length established in this manner persisted through the nestling period. Egg‐size effects on feather growth have not been detected in experimental studies on other birds, suggesting that effects of the magnitude seen in Razorbills and Thick‐billed Murres might reflect evolutionary priorities in the post‐hatching development of intermediate auks.